MTN POOR CUSTOMER EXPERIENCE

I have a contract with MTN for almost 7 years now. We had an agreement that I should pay the money using the account I was given at MTN store every end of the month. While I was paying to that account they phoned me giving demanding my account number so that I can be debited on the 1st of every month. The debit order continued for 5 months then it stopped. Now the MTN credit won't be removed because it says I owe R878. I can't even take a bank loan because my credit says I owe MTN R878. I tried contacting them with email and calling them but got no response. So should I be punished as a customer for their mistake. They don't even communicate back